Expert: Mark Sundberg - 1/7/2009

Question
pls i'm designing a pre-school floor plan as a school project and i need detailed diagrams of roof types and sections; especially flat roofs suited for tropical regions. i'm attaching a roughly drawn-up copy of my plan  sorry about the roughness i didn't have enough time to scan a better copy

Answer
Aloha Aisha,

Well, no attachment so I can't comment on that. It seems to be a hit and miss sort of thing.

Anyway, to your question or request. Best I can offer is to point you to Architectural Graphic Standards. It's sort of the bible for architecture. It should show you just about every framing system commonly used.

One thing, though, you should be aware that roof framing is highly dependent on both the shape of the roof and the materials to be used. A steeply pitched wood framed hip roof would frame entirely differently from a relatively flat metal framed shed roof.

I'd also question your apparent assumption that flat roofs are suitable in tropical regions. Typically, flat roofs are a response to issues other than climate or weather. It's often a matter of what materials are available, whether the roof is to be habitable and cost, both in materials and labor. Flat roofs are pretty easy to build and so relatively inexpensive. They also often leak badly. This is why they are generally used, at least in indigenous construction, in arid areas, there's not much rain to leak in the first place and what there is, one wants to capture every drop possible for later use. Those roofs are often more for shade than anything else. Of course, that ignores the fact that desert downpours are often some of the most violent there are.

Then again, it would depend a lot on whether you are talking about a wet tropics or a dry tropics. In a dry tropics, a flat roof is probably OK, might even be a good choice. In a wet tropics, it tends to be a bad choice because you want to get rid of the water as fast as you can and the easiest way to do that is to have it run off a steep slope. Preferably one that extends below the floor line so the splatter doesn't come back into the building. There's also the advantage of having a high interior volume for the warm moist air humans in the tropics will generate to collect in for exhausting.

If there are good reasons for a flat roof, then they frame pretty much the same. There will be a roof deck, plywood, metal panel, etc. which supports the roofing material itself, what keeps the outside out and the inside in and which is supported on some sort of rafters or beams. The rafters (if any) would be supported either on beams and girders or bearing walls. The beams and girders would then be supported on columns and bearing walls or pilasters (thickened portions of a wall).

For a small structure, most likely there would be just rafters and maybe a beam or two. The rafters would be spaced at some small interval, usually 16" or 24" in the US. Not sure about metric equivalents though the few occasions I've worked overseas, they usually just convert the US modules to the nearest mm. so those would be 406mm and 610mm respectively. Usual spans for rafters are between 8' to 24' though some engineered wood and metal systems can span up to 40'. It's becoming common practice to provide positive anchorage of rafters to their bearing, so that uplift forces from wind does not tear the roof off. You can look at the Simpson Strong-Tie website for examples. This anchorage also applies to beams and girders and you can also see examples of those on the Simpson Strong-Tie website. Beyond this point, the variations in support for beams and girders gets way too much to cover it all. In essence, it's roof deck to rafters (or purlins), rafters to beams and walls, beams to girders, columns and walls and girders to columns and walls. The column and walls are supported on footings in the ground.

If you have specific questions, I might be able to be of more assistance.